1. Field of the Invention
The present invention relates to propylene polymers. In particular, the present invention concerns high-stiffness propylene homopolymers and propylene copolymers containing a high-stiffness polypropylene matrix. The invention also relates to a process for preparing propylene homo- and copolymers.
2. Description of Related Art
Propylene (PP) homo- and copolymers have excellent resistance to heat and chemicals as well as attractive mechanical properties, such as stiffness and impact resistance. However, processing of polypropylene by, e.g., injection moulding, thermoforming or blow moulding, to form thin-walled containers has resulted in products having insufficient stiffness, transparency and cycle time. This is caused by the semi-crystalline nature of polypropylene.
In the prior art it has been proposed to improve the stiffness, transparency and cycle time of moulded polypropylene by blending the polymer with various nucleating agents such as dibenzilidene sorbitol (DBS), sodium benzoate or di(alkylbenzilidene)sorbitol. These traditional nucleating agents tend to bleed out from the polymer composition during processing and many of them give rise to fumes with an offensive smell. As a solution to these problems, it has been suggested in the art to use vinyl compounds, such as polymers of vinyl cycloalkanes and 3-methyl-1-butene, as nucleating agents in the form of propylene copolymers or polypropylene compounds, cf. EP Patent Specifications Nos. 0 151 883, 0 152 701, 0 206 515, 0 368 577 0 369 658 and0 417 319. EP Patent No.0 152 701 discloses prepolymerization of Ziegler-Natta catalysts with vinyl cyclohexane to provide a polymer slurry which is washed and distilled to produce a vinyl cyclohexane powder containing the active catalyst. The prepolymerized catalyst composition is then used for polymerization of propylene to form a propylene copolymers with improved stiffness and having a high degree of crystallinity and a high crystallization temperature.
There are some major problems associated with the prior art solutions using polymerized vinyl compounds for nucleation of polypropylene. Thus, the products contain impurities in the form of unreacted monomers and extensive washing of the product has to be carried out before the catalyst can be used. These washing steps will reduce that activity of the catalyst. In fact, the whole work-up of the prepolymerized catalyst, including separation of the catalyst from the polymerization medium, washing and drying, will cause extra costs and impair the activity of the catalyst. Further, if very high stiffness properties are desired, they are not achieved with these polymers without adding specific fillers or additives.
Finally, it should be pointed out that it is known in the art to carry out prepolymerization in a medium comprising a viscous substance and to continue the prepolymerization with vinyl cyclohexane or to carry out the prepolymerisation in the presence of 4-methyl-1-pentene monomer and a weakly coordinating donor, MTBE (cf. Finnish Patent No. 95387). Due to the fact that, e.g., no washing, drying, sieving and transferring steps are needed, the catalyst activity is maintained.
It is an object of the present invention to eliminate the problems relating to the prior art and to provide nucleated propylene homopolymers, propylene copolymers comprising propylene random and heterophasic copolymers.
In particular the present invention aims at providing high-stiffness propylene homo-polymers as well as propylene copolymers.
The present invention also aims at providing a process by which it is possible to prepare modified catalysts providing excellent nucleation of propylene polymers and containing essentially no reactant or solvent residues which would impair the long-term activity of the catalysts. In particular, it is an object of the present invention to provide a process for increasing the stiffness of nucleated polypropylene. Further, in case of propylene homopolymers, as a final product or as polymer matrix, the amount of xylene solubles comprised in the polymer will be reduced.
Further, it is an object of the present invention to provide extruded and moulded products comprising the present propylene homo- and copolymer compositions.
These and other objects, together with the advantages thereof over known processes and products, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
The invention is based on providing a specific kind of catalyst system of Ziegler-Natta type useful for polymerization of propylene, optionally together with comonomers, for producing propylene polymers having high-stiffness properties. The catalyst is modified by polymerizing a vinyl compound in the presence of said catalyst. The modification takes place in a medium which is a liquid or a highly viscous hydrocarbon medium and which does not dissolve the polymerized polymer. Further, the modification is accomplished in such a way that all or practically all vinyl compound used for modification is consumed during polymerization. To reach that aim, the polymerization is continued at elevated temperature until the concentration of the unreacted vinyl compounds in the catalyst composition after polymerization is about 0.5 wt-%, preferably less than 2000 ppm by weight and in particular 1000 ppm by weight or less. A sufficient amount of the initial reactant is a maximum of 10 times, preferably below three times the weight of the catalyst. The modification is carried out before any conventional, usually continuous prepolymerization with an olefinic monomer, to ensure that the polymerization of the vinyl compound is complete.
As a result of these features, the amount of reactant residues in the modified catalyst composition is small, and in the final propylene polymer it is below limits of determination using the Gas Chromatography-Mass Spectrometry (GC-MS) method, which is less than 0.01 ppm by weight. Since the reaction medium contains only very small amounts of unreacted reactant residues or dissolved polymer residues, no washing of the modified catalyst composition is needed before the catalyst is fed to polymerization.
According to the present invention the stiffness of the polymer can further be improved if the above catalyst modification is carried out in the presence of external donor, particularly a strongly coordinating external donor.
Further, according to the invention an improved stiffness/impact strength balance can be obtained for heterophasic polypropylenes. Accordingly, either the stiffness is higher, while the impact strength remains on the desired level, or the impact strength is better, while the stiffness remains on the desired level, or both the stiffness and the impact strength are slightly better, than if the modification were carried out in the absence of the strongly coordinating external donor.
According to another aspect of the present invention, in the case of propylene homopolymers, the amount of xylene solubles formed in the nucleated polypropylene is reduced. Thus, it has been found that the amount of xylene solubles can be radically reduced when the catalyst modification is carried out according to the invention by polymerizing the catalyst with a vinyl compound in the presence of a strongly coordinating donor. As a result, a high-stiffness propylene homopolymer is obtained exhibiting a xylene solubles concentration of less than 1.5%. Propylene homopolymer can be a final product or a polymer matrix, into which other comonomers, such as xcex1-olefins or ethylene, can be combined.
More specifically, the propylene polymer according to the invention is mainly characterized by a nucleated high-stiffness propylene polymer composition obtainable by homopolymerization or copolymerization of propylene in the presence of a catalyst system comprising a catalyst component, a cocatalyst component, and an external donor, the procatalyst component of the catalyst system containing magnesium, titanium, halogen and an electron donor, said catalyst being modified by polymerizing it with a vinyl compound of the formula 
wherein R1 and R2 together form a 5 or 6 membered saturated or unsaturated or aromatic ring in the presence of a cocatalyst and a strongly coordinating external donor.
The process according to the present invention is characterized by a process for producing a high-stiffness propylene polymer nucleated with a polymeric nucleating agent containing vinyl compound units, comprising the steps of modifying a catalyst system, the procatalyst component of which contains magnesium, titanium, halogen and an electron donor, by polymerizing a vinyl compound of the formula wherein R1 and R2 together form a 5 or 6 membered saturated or unsaturated or aromatic ring, at a weight ratio of the vinyl compound to the catalyst amounting to 0.1 to below 3, in the presence of said catalyst, a cocatalyst and a strongly coordinating external donor, by carrying out the modification in a medium which does not essentially dissolve the polymerized vinyl compound and by continuing the polymerization of the vinyl compound until the concentration of unreacted vinyl compounds is less than about 0.5 wt-%, to produce a modified catalyst composition, said modification being carried out essentially before any prepolymerization step of the catalyst with an olefinic monomer, and polymerizing propylene optionally together with comonomers in the presence of said modified catalyst composition.
The invention achieves a number of considerable advantages, some of which were already discussed above. In particular it can be noted that the present propylene polymers are characterized not only by high stiffness but also by high crystallinity and high crystallization temperature. In comparison to conventional polypropylene the present polymers exhibit good mechanical properties, such as high modulus, high heat resistance and water vapour barrier. Further, an improved balance between stiffness and impact strength of block copolymers can be obtained. Very good and consistant nucleation improves clarity in a better way than with conventional nucleating agents. Nucleation dominates effect from different pigments; this means consistent shrinkage and warpage in multicoloured parts. The crystallinity is influenced by the high isotacticity (preferably  greater than 98 %) of the homopolymer and by the effective nucleation with the polymerised vinyl compounds.
According to the invention propylene polymers having from at least 3%, preferably from at least 5%, up to 10%, or even up to 15%, higher stiffness can be obtained than without using a donor in the modification step. Propylene heterophasic copolymers of the invention exhibit from 2% up to 20% higher impact strength than would be obtainable by polymerization of propylene and ethylene/propylene with a catalyst, which is modified in the absence of a strongly coordinating external donor.
With the present invention the level of xylene solubles can be decreased to the same level as without modification with a vinyl compound. The particular benefit of the increased stiffness is that it is possible to produce materials, both homopolymers and heterophasic PP copolymers, which exhibit higher stiffness than conventional nucleated products. This feature is of particular importance for the manufacture of pipes, such as smooth solid wall pipes, fittings, and pipe system details, e.g. valves, chambers and manholes, for indoor or buried sewage, multilayer pipes and fittings for indoor or buried sewages, and structured wall pipes and fittings for buried sewage.
In addition to pipes, the present compositions can be used in any kind of polymer articles. Particular advantages are obtained by applying the compositions to the manufacture of appliances, automotive parts, cups, pails, containers, caps or closures. The new material can also be used in various cable and tube applications.
According to one preferred embodiment, which is described in more detail below, the donor used in the modification step is the same as used in propylene polymerization.
Modification of the catalyst by polymerization of vinyl compounds in the liquid or highly viscous medium described above will reduce production costs because higher capacities can be used and no wastes are formed. Reliability of the catalyst activity is good because this modification is a part of the polymer production and no kind of transferring, drying or sieving is needed.
A further advantage of the invention is that no other nucleating agent is needed and/or the amount of nucleating agents added can be reduced. Further, the amount of donor used in the propylene polymerization can be reduced. Also improvements in operability of the process have been noticed.
Because the final products do not contain harmful residues of the vinyl compounds, the propylene polymers manufactured by the present invention have a broad range of application.
By using the modified catalyst compositions of the present invention, propylene polymers can be prepared having a Melt Flow Rate (MFR2) of 0.01 to 1500 g/10 min (measured by ISO Standard 1133, at 230xc2x0 C., 2.16 kg load) and a Tcr of over 7xc2x0 C. higher than the Tcr of the corresponding non-nucleated polymer. The crystallinity of propylene homopolymers is generally over 48%.
Next, the invention will be more closely examined with the aid of the following detailed description with reference to the attached drawing.